Redox activity of surface oxygen anions in oxygen-deficient perovskite oxides during electrochemical reactions

David N Mueller,William C Chueh,Hendrik Bluhm,Michael L Machala

doi:10.1038/ncomms7097

Abstract

Surface redox-active centres in transition-metal oxides play a key role in determining the efficacy of electrocatalysts. The extreme sensitivity of surface redox states to temperatures, to gas pressures and to electrochemical reaction conditions renders them difficult to investigate by conventional surface-science techniques. Here we report the direct observation of surface redox processes by surface-sensitive, operando X-ray absorption spectroscopy using thin-film iron and cobalt perovskite oxides as model electrodes for elevated-temperature oxygen incorporation and evolution reactions. In contrast to the conventional view that the transition metal cations are the dominant redox-active centres, we find that the oxygen anions near the surface are a significant redox partner to molecular oxygen due to the strong hybridization between oxygen 2p and transition metal 3d electronic states. We propose that a narrow electronic state of significant oxygen 2p character near the Fermi level exchanges electrons with the oxygen adsorbates. This result highlights the importance of surface anion-redox chemistry in oxygen-deficient transition-metal oxides.

Highlights

Surface redox-active centres in transition-metal oxides play a key role in determining the efficacy of electrocatalysts
The electron-transfer step has been proposed to be rate determining[3,4,5]. Such a two-way traffic of ions and electrons is significantly more complex and less understood than electrochemical reactions occurring at the interface between a liquid electrolyte and a metal or semiconductor electrode, where electrons are the only species transferred between the redox species in solution and the solid (Fig. 1b)[1,6]
Chemical intuition arising from the redox behaviour of TM cations suggests that they are the redox partner to molecular oxygen during electrochemical reactions

Summary

Introduction

Surface redox-active centres in transition-metal oxides play a key role in determining the efficacy of electrocatalysts. We probe the surface electronic structure of (La,AE)TMO3 À d electrodes with Fe and Co as TM cations and Ca, Sr or Ba as the alkaline-Earth (AE) substituents under oxygen incorporation and evolution reaction conditions at elevated temperatures.

Results

Conclusion